The use of downhole motors in the drilling and completion of wells is well known in the art. However, until recently, due to certain shortcomings, such motors have been used almost exclusively for special situations (e.g. drilling of directional wells) and have found only limited use in the everyday drilling of straight, vertical wells. Some of these shortcomings have been overcome by recent developments in drill bits and in certain materials used in the construction of the motors, themselves.
Two types of fluid-powered downhole motors (mud motors) are in general use. Most popular in the United States is the Moineau, positive displacement type. The turbine motor is widely used in the Soviet Union, the North Sea and the Middle East. Electrically powered motors are not greatly used in drilling today because of difficulties in handling the power cable. The instant invention is applicable to all three types of downhole drilling motors.
However, problems still remain which have to be overcome before downhole motors will find widespread acceptance in routine straight-hole drilling operations. Probably the most pressing of these problems lies in the inability to apply sufficient weight-on-bit when using known downhole motors to achieve satisfactory straight-hole drilling and to optimize the drilling rate of penetration. This problem results from the fact that the outer case of the motor is coupled directly into and forms an integral portion of the drill string. The outer motor case does not have the stiffness required throughout its length and hence to bend when it transmits the necessary drill collar weight onto the bit to carry out the desired drilling operation. In other words, if sufficient drill collar weight is applied through the length of the motor case, the compressive and bending forces on the case become excessive and can cause severe damage to the motor or crooked holes. Also, since the motor case is coupled directly into the drill string, the connection between the motor and the drill bit is such that shock loads experienced during drilling are transmitted directly to the motor thereby causing undue wear on the bearings and seals thereof which, in turn, lead to early failure.
Further, existing downhole motors do not have the torque or horsepower required to rotate the drill bit when heavily weighted as is usually required for economic straight-hole drilling. It has been proposed to stack several power sections of a motor in tandem to increase the horsepower output of the motor but again the flexibility of the motor case severely restricts the number such sections that can be stacked in the drill string. Still further, the drilling fluid which passes through the motor to drive the rotor thereof creates a pressure drop across the motor which results in a downward pressure or thrust which normally is transmitted to the drill bit when the drill string is lowered onto the bottom of the wellbore. When this downward hydraulic thrust is exactly balanced by the weight-on-bit, there is no vertical load on the bearings in the motor. This no-load or "neutral" condition is ideal for long bearing life and, although recommended by most downhole motor suppliers, is extremely difficult to achieve and maintain during drilling operations. This commonly results in uneconomically slow rate of penetration or in excessive motor wear.
Present motor characteristics, also, generally limit both the rate of flow of drilling fluid through the motor and the pressure drop (mentioned above) which the motor can handle without damage. This limited rate of flow is usually less than that desired or required for the proper cooling of the bit or for proper cuttings removal in most straight-hole drilling operations.